VM Freeze on `capsem-pty-agent` Exit

During an Antigravity conversation inside a Capsem VM, agy executed `/run/capsem-pty-agent --help`. This resulted in the machine freezing, and the only way out was to go back to the dashboard to pause and restart the VM.

I could reproduce the bug manually like this:
* Create a new VM and log into the terminal
* Open tmux, check that the first pane is active (e.g. run `pwd`)
* Go to the second tmux pane, and type `/run/capsem-pty-agent --help`.
* The VM immediately hangs.

I attached an AI-generated bug report below. If this is not working as intended, I'm happy to work on a PR, at least for the basic fixes to `capsem-pty-agent` such as adding argument parsing and locks.

[bug-report-pty-agent-freeze.md](https://github.com/user-attachments/files/32017843/bug-report-pty-agent-freeze.md)

Start by reproducing `/run/capsem-pty-agent --help` in a new VM with the stated tmux setup, then locate the `capsem-pty-agent` entry point and inspect how this invocation handles arguments and concurrent terminal activity. Done means the help command exits without freezing the VM and the reproduction steps no longer require pausing or restarting it.

Written by the indexing model from the issue text.